You're not even at the school age yet to understand the demands of that, never mind the teen years.
As someone whose children are a little older can I just mention that you're on the relatively easy bit?
I agree with @pecanroll and @Isonthecase
It's no coincidence that posts like this are nearly always from mothers of children of aged primary and under.
@Soitis83 During the primary school years you will be running yourself ragged running this child or that child to and from after school activities, playdates and birthday parties. Then there is the expense of school uniforms, school trips, requests from school for monetary donations and gifts for fundraising activities.
Then secondary school - supporting your DC to do their homework, being a shoulder to cry on when their best friend isn't talking to them any more, potentially bullying, GCSE stress x 4. A level stress x 4, UCAS x 4, the expense of providing enough technology for them to do homework (4 laptops, 4 mobile phones), then the biggest expense of them all - topping up student loans when they are at university.
You can afford 4 children at this age. Do you think that you could afford four teenagers?
Also, the drudgery. All the cooking, washing and cleaning. Even if your hsband pulls his weight it is a lot of work.
Never underestimate how hard it is to parent even the easiest of teenagers. They need you far more on an emotional level than small children where a simple hug and a kiss makes them feel better.
you never regret the children you have
Actually, that's not true. It's a taboo subject, but there have been several threads on mumsnet from posters who would disagree with you.
I love raising children. Not a hobby by any means, a life goal for me personally.
I hope that isn't the only goal you have in life. There must be other worthwhile things you want to do?
I'm out the other side, but I found the primary school years the easiet, and the secondary school years the hardest. There is no way I would have wanted to endure that 4 times.
Can I suggest that you read the following topics to get an idea of what can lie ahead before you make any rash decisions:
Teenagers
Secondary education
Further education
Higher education
Also, you haven't mentioned what your DH thinks. Does he even want any more children?